A PHYTOSOCIOLGICAL CASE STUDY OF SOME WEEDS ASSOCIATED WITH CROP SESAME (SESAMUM INDICUM L.) IN THE SEMI ARID REGION OF NORTH-WESTERN DESERT OF RAJASTHAN.

Weeds are excellent examples of successful struggle for existence. Weeds are adapted by their abundant seed production, dispersal, varied seed dormancies, high competitive potentiality and ability to spread rapidly. Weeds have unique potentialities for adaptation; they thrive in almost any environment and adjust themselves to the changed conditions. Survival is difficult in arid region, where conditions are most inhospitable and impose severe constraints on plant life. Once a certain weed species is introduced, its abundance or scarcity is determined largely by the degree of competition offered by the crop and the prevailing environment. In agro-ecosystem weeds are harmful because they interfere with agricultural operations, which increase labour, compete with crop for water, nutrients, space, soil, light etc., and finally reduce the crop yield. Not only this, weeds have enormous seed production, variety of seed dormancies, first to grow and multiply under stress conditions and power of vegetative multiplication. They show rapid spreading and a deep penetrating root system, resist drought, start flowering and fruiting much earlier than crop and finally produce seeds to continue their progeny .Weeds not only rob the crops of nutrients but also the moisture which is most important ingredient in rain fed cropping. In present study the crop weed association was composed of 22 species. High frequency percent was shown by Tribulusterrestris. Farestiahamiltonii, Tribulusalatus. The relative biomass of crop and weeds were somewhat in proportion to relative densities. The net production of crop was higher than that of weeds. The period between 30 to 60 days after sowing was most critical stage for crop weed competition.